In general, the present invention relates to a method of producing a lithographic mask (reticle) for use in the semiconductor industry. In particular, the invention pertains to a particular combination of process steps useful in preparing optically imaged high performance photomasks. The optical imaging of the photomask makes use of a deep ultraviolet (DUV) photoresist in combination with at least one antireflective coating (ARC). The DUV photoresist is imaged using an optical direct write continuous laser mask writing tool.
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components, such as in the fabrication of semiconductor device structures. The miniaturized electronic device structure patterns are typically created by transferring a pattern from a patterned masking layer overlying the semiconductor substrate rather than by direct write on the semiconductor substrate, because of the time economy which can be achieved by blanket processing through a patterned masking layer. With regard to semiconductor device processing, the patterned masking layer may be a patterned photoresist layer or may be a patterned xe2x80x9chardxe2x80x9d masking layer (typically an inorganic material or a high temperature organic material) which resides on the surface of the semiconductor device structure to be patterned. The patterned masking layer is typically created using another mask which is frequently referred to as a photomask or reticle. A reticle is typically a thin layer of a metal-containing layer (such as a chrome-containing, molybdenum-containing, or tungsten-containing material, for example) deposited on a glass or quartz plate. The reticle is patterned to contain a xe2x80x9chard copyxe2x80x9d of the individual device structure pattern to be recreated on the masking layer overlying a semiconductor structure.
A reticle may be created by a number of different techniques, depending on the method of writing the pattern on the reticle. Due to the dimensional requirements of today""s semiconductor structures, the writing method is generally with a laser or e-beam. A typical process for forming a reticle may include: providing a glass or quartz plate, depositing a chrome-containing layer on the glass or quartz surface, depositing an antireflective coating (ARC) over the chrome-containing layer, applying a photoresist layer over the ARC layer, direct writing on the photoresist layer to form a desired pattern, developing the pattern in the photoresist layer, etching the pattern into the chrome layer, and removing the residual photoresist layer. When the area of the photoresist layer contacted by the writing radiation becomes easier to remove during development, the photoresist is referred to as a positive-working photoresist. When the area of the photoresist layer contacted by the writing radiation becomes more difficult to remove during development, the photoresist is referred to as a negative-working photoresist. Advanced reticle manufacturing materials frequently include combinations of layers of materials selected from chromium, chromium oxide, chromium oxynitride, molybdenum, molybdenum silicide, and molybdenum tungsten silicide, for example.
As previously mentioned, the reticle or photomask is used to transfer a pattern to an underlying photoresist, where the reticle is exposed to blanket radiation which passes through open areas of the reticle onto the surface of the photoresist. The photoresist is then developed and used to transfer the pattern to an underlying semiconductor structure. Due to present day pattern dimensional requirements, which are commonly less than 0.3 xcexcm, the photoresist is preferably a chemically amplified DUV photoresist. In the making of the reticle itself, a chemically amplified DUV photoresist has been used in combination with a direct write electron beam writing tool. Additional work has been done recently using a direct write continuous wave laser tool available under the trade name ALTA(trademark) from ETEC Systems Inc., Hillsboro, Oreg.
Preparation of a photomask/reticle is a complicated process involving a number of interrelated steps which affect the critical dimensions of a pattern produced in the reticle, and the uniformity of the pattern critical dimensions across the surface area of the reticle. By changing various steps in the reticle manufacturing process, the reproducibility of the manufacturing process itself may be altered, including the processing window. Processing window refers to the amount process conditions can be varied without having a detrimental outcome on the product produced. The larger the processing window, the greater change permitted in processing conditions without a detrimental affect on the product. Thus, a larger process window is desirable, as this generally results in a higher yield of in specification product produced.
Various efforts are made within the industry to improve the reliability of manufacturing processes by improving individual process steps; however, when a production process involves a number of interrelated process steps, alteration of an individual process step may have an unexpected result on other interrelated process steps.
The reticle manufacturing process steps generally include the following, where the initial substrate used to form the reticle is a silicon oxide-containing base layer having a layer of a metal-containing (typically chrome) mask material applied thereover. An inorganic antireflective coating (ARC) or an organic ARC, or a combination of inorganic and organic ARC layers may be applied over the surface of the chrome mask material. A photoresist layer is then applied over the antireflective coating. The photoresist is typically an organic material which is dissolved or dispersed in a solvent. The solution or dispersion of photoresist is typically spin coated onto the surface of the photomask fabrication structure. Typically, the photoresist is applied over an ARC layer on the fabrication structure surface. Some of the solvent or dispersion medium is removed during the spin coating operation. Residual solvent or dispersion medium is subsequently removed by another means, typically by baking the fabrication structure, including the photoresist layer. This step is commonly referred to as xe2x80x9cPost Apply Bakexe2x80x9d or PAB. The photoresist is then exposed to radiation (imaged), to produce a pattern in the photoresist layer, typically by a direct write process when the pattern includes dimensions which are less than about 0.3 xcexcm or less. After exposure, the substrate including the photoresist layer is baked again. The second baking is typically referred to as xe2x80x9cPost Exposure Bakexe2x80x9d or PEB. The photoresist is then developed either using a dry process or a wet process, to create the pattern having openings through the photoresist layer thickness. Once the photoresist is xe2x80x9cpatternedxe2x80x9d so that the pattern openings extend through the photoresist layer to the upper surface of an ARC layer, or to a surface beneath an ARC layer, the pattern in the patterned photoresist is transferred through the chrome-based mask layer and any remaining layers overlying the chrome layer, for example, typically by dry etching.
U.S. patent application, Ser. No. 09/848,859, filed May 3, 2001, titled: xe2x80x9cOrganic Bottom Antireflective Coating For High Performance Mask Making Using Optical Imagingxe2x80x9d now U.S. Pat. No. 6,605,394, and assigned to the assignee of the present invention, describes a reticle fabrication process in general. This patent application is hereby incorporated by reference in its entirety. As disclosed in the ""859 application, there are a number of problems encountered in trying to produce a photomask/reticle when the photomask pattern exhibits critical dimensions of less than 0.3 xcexcm. Further, producing a reticle where pattern critical dimensions are uniform across the entire reticle surface requires careful control of process variables in each step of the reticle manufacturing process. For example, the developed (patterned) photoresist on the surface of the underlying substrate, prior to pattern transfer, frequently exhibits a xe2x80x9cfootxe2x80x9d at the bottom of the pattern profile, where the photoresist layer interfaces with an underlying ARC layer on a chrome-containing surface, despite the presence of the underlying ARC layer (which is typically a chrome oxynitride material). The foot is not uniform in size across the reticle substrate surface because the basisity changes somewhat randomly across the substrate surface. Since the foot is variable, it makes it difficult to do the metrology which is used to determine whether the finished reticle will meet dimensional requirements.
Some imaged and developed positive tone photoresists exhibit a xe2x80x9ctxe2x80x9d-top profile. In addition, the surface of the patterned photoresist layer typically exhibits standing waves, due to reflections which occur during the direct writing on the photoresist layer, despite the presence of the underlying ARC layer.
In their 1992 paper in Microelectronic Engineering (Vol. 17 (1992) 275-278) Gilles Amblard et al. describe how the development of chemically amplified (CA) resist systems has been the most successful approach to meeting the challenge of high resolution and high speed, posed by X-Ray, Electron-Beam or Deep UV lithography. However, they discovered that pattern profile abnormalities appear which limit the use of a negative resist. Even though the correct exposure dose is applied throughout the thickness of the desired pattern, an aqueous developer dissolves the bottom part of the resist in contact with or near the underlying substrate. Fissures as thick as 0.1 to 0.2 xcexcm were observed in the pattern at the interface with the substrate, resulting in a loss of adhesion in fine patterns. The problem was observed for resists imaged and developed on both spin on glass (SOG) and aluminum substrates.
Japanese Patent No. 10048831 assigned to Sony Corp and granted Feb. 20, 1998, relates to patterning of a chemical amplification-based resist film on a film which is to be patterned. The composition of the film to be patterned is not specified in the English abstract of the Japanese patent. The formation process comprises: (a) covering the film to be patterned with a protective coating consisting of chalcogen except sulphur; (b) depositing the chemical amplification-based resist film on the protective coating; (c) applying selective exposure, baking after exposure, and development of the chemical amplification-based resist film to form a resist pattern; and (d) selectively removing the exposed portion of the protective coating. The advantage is said to be that the surface of the film to be patterned is previously passivated by the protective coating. This prevents diffusion of active species between the chemical amplification-based resist film and the film to be patterned and prevents the active species from a decrease in its concentration around the interface against the film to be patterned. The resulting resist pattern is said to have xe2x80x9cno unusual shapexe2x80x9d
In U.S. Pat. No. 5,723,237 to Kobayashi et al., issued Mar. 3, 1998, the inventors disclose a method for determining baking conditions for resist pattern formation through development of unexposed trial resist films. In particular, resist patterns which minimize the standard deviations of critical dimensions within a plate and between plates (namely which minimize the critical dimensions) are formed by a specialized method. The method permits determining conditions for resist pattern formation which includes a film forming process, a resist film baking process, an exposing process, and a developing process. The method includes preparing a plurality of plates, each having a resist film formed thereon, baking the resist films on the plates, with each plate having a different baking condition. Without performing an exposure of the film to pattern imaging, the baked films are subjected to a dissolving treatment to induce partial dissolution of the resist films. The data derived is used in determining the temperature condition for baking a prospective resist film in a manner which minimizes the change in uniformity of the remaining resist thickness (after partial dissolution) across the plate.
FIG. 1 (not taken from the ""237 patent, but published by the assignee of the patent, Hoya) shows a graph 100 of the thickness uniformity across a reticle plate, for a photoresist film having an initial thickness of about 5,000xc3x85, after subjection to a dissolving treatment which reduced the mean film thickness. Axis 120 of the graph 100 shows the range of film thickness variation across the reticle plate as a function of the PAB temperature. The thickness uniformity (of the remaining film) is shown as a function of the post apply bake (PAB) temperature on graph axis 110. The temperature measured was that of a hotplate under the reticle plate rather than the temperature of the photoresist itself, however. The data is shown for films of a DUV photoresist (DX1100P/AR3 which is available from AZ Clariant Corp. of Somerville, N.J.). The DUV photoresist films were baked at a given PAB temperature for time periods of 9 minutes (curve 112), 12 minutes (curve 114), or 15 minutes (curve 116). FIG. 1 also shows the rate of change of the normalized film thickness as a % of the original film thickness on graph axis 130, as a function of the PAB temperature illustrated on graph axis 110 for films baked for various time periods. A bake period of 9 minutes is shown on curve 102, 12 minutes is shown on curve 104, and 15 minutes is shown on curve 106. Clearly the PAB temperature is important with respect to development of a photoresist both in terms of the development rate and the uniformity of development across the surface of the photoresist.
In their paper entitled: xe2x80x9cImprovement of Post Exposure Delay Stability Of Chemically Amplified Positive Resistxe2x80x9d, presented at the SPIE Symposium on Photomask and X-Ray Technology VI, Yokohama Japan, September 1999 (SPIE) Vol. 3748.0277-786X/99, Kohji Katoh et al describe the development of a novolak-based chemically amplified positive resist for next generation photomask (below 0.18 xcexcm) fabrication. The resist is said to prevent footing at the base of a profile by the use of a hydrophilic polyphenol compound. The resist was used to make a well defined 0.25 xcexcm line-and-space pattern on a CrOx substrate at a dose of 4.0 uC/cm2. The advanced high acceleration voltage (50 kV) E-beam writer HL-800M was developed to provide better critical dimension control. However, the high acceleration voltage lowers the sensitivity of resists. To compensate, a chemically amplified resist was needed. The resist developed includes four components: a novolak matrix resin, a polyphenol compound, an acid generator, and a dissolution inhibitor.
In their paper xe2x80x9cEnhancement or Reduction of Catalytic Dissolution Reaction in Chemically Amplified Resists by Substrate Contaminantsxe2x80x9d (published in IEEE Transactions On Semiconductor Manufacturing, Vol. 12, No. 4, November 1999), Choi Pheng Soo et al. describe the chemical interaction of resist and substrate at the interface, which modifies the dissolution reaction, and has degraded sidewall profile of the resist features. Depending on the nature of the residue on the substrate, the xe2x80x9cbottom pinchingxe2x80x9d (BP) effect and footing are observed, especially for negative chemically amplified (CA) resists. The BP effect is observed for CA resist on top of an organic bottom antireflection coating (BARC). The BP is attributed to the acid generated from the underlying organic BARC. With optimization on soft bake temperature of BARC, the BP effect is said to be eliminated.
European Patent Application No. EPO 987 600 A1 of Timothy G. Adams et al: assigned to Shipley Company LLC, published Mar. 22, 2000, describes new light absorbing crosslinking compositions suitable for use as an antireflective composition (ARC), particularly suitable for short wavelength imaging applications such as 193 nm. The ARCs are preferably used with an overcoated resist layer (i.e. as bottom layer ARCs) and in general comprise ARC resin binders that can effectively absorb reflected sub-200 nm exposure radiation. In particular, the antireflective composition comprises a resin binder that has phenol groups. The phenol groups are described as directly pendant from the resin backbone of the antireflective composition resin.
The above descriptions pertain to the use of chemically amplified photoresist on semiconductor substrates, or to the use of a chemically amplified photoresist in combination with electron beam lithography to produce a reticle. The present invention is different in that it pertains to the use of an optical system, a direct write continuous wave laser, to image a chemically amplified photoresist which is used to transfer a pattern to a photomask (reticle). However, many of the problems described above are experienced in producing a reticle using an optical imaging system in combination with a chemically amplified photoresist.
FIG. 2A shows a schematic of a cross-sectional view of a prior art starting structure 200 used to form a reticle, including, from bottom to top, a quartz substrate 202, overlaid with chrome-containing layer 204, overlaid with an ARC layer 206, and a positive tone photoresist layer 208. As shown in FIGS. 2B and 2C, after patterning of the photoresist layer 208 using an electron-beam writing tool, there is often a xe2x80x9cfootxe2x80x9d 210 extending from the lower portion of patterned photoresist layer 208 toward the surface 216 of ARC layer 206. The presence of a foot (feet) 210 makes it difficult to maintain control of the critical dimensions during subsequent etch transferring of the photoresist pattern through the ARC layer 206 and chrome containing layer 204. The foot also impacts the metrology capabilities of the lithographer.
FIG. 2C, which is an enlargement (from FIG. 2B) of a portion of the patterned photoresist layer 208 (with underlying ARC layer 206), shows a line 207 which exhibits xe2x80x9ctxe2x80x9d-topping 213 in the upper portion of line 207, feet 210 at the base of line 207, and ripples (standing waves) 214 on the sidewall 211 surfaces 212 of line 207. The xe2x80x9ctxe2x80x9d- topping 213 is believed to be caused by contamination/reaction which occurs at the upper surface of the photoresist layer during processing prior to development of the pattern. The standing waves 214 are generated by reflected radiation within the photoresist material, which occurs during the direct writing of the pattern into photoresist layer 208 by the electron-beam writing tool. The ARC layer 206 helps reduce the standing wave effect by reducing reflection back from underlying layers and device features into the photoresist layer 208, but standing waves are generated in varying degrees depending on the imaging system and the material composition of the particular photoresist. When the photoresist is a chemically amplified photoresist, transparency of the photoresist material is particularly high throughout the entire direct writing process; this results in increased reflectivity (greater than that for earlier i-line novolak photoresists), which increases the formation of standing waves 214.
FIG. 2D illustrates a side view schematic of a developed photoresist which is a negative tone DUV chemically amplified photoresist. The developed resist exhibits xe2x80x9cpinchingxe2x80x9d at the base of a line pattern, as shown in FIG. 2D. A line 221 in negative tone patterned photoresist 228 exhibits standing waves 224 on sidewall surface 222 and a narrowing or xe2x80x9cpinchingxe2x80x9d at the base 230 of line 221. The pinching occurs because, in a negative tone resist 228, the irradiated portion of the resist reacts (typically crosslinks) to form a polymer which is insoluble in the developing solution during development of the pattern. Photoresist 228 sidewall surface 222 is undercut as indicated by arrows 232 during development of the irradiated pattern including line 221.
It is readily apparent that it would be highly desirable to have a method of making a photomask which provides features having critical dimensions of 0.3 xcexcm or less, where the uniformity of the critical dimensions is maintained across the entire surface of the photomask. To accomplish this, it is necessary to have a method of producing a patterned, developed photoresist which is imaged and developed uniformly across the photomask surface. The developed pattern profile of the photoresist needs to exhibit minimal surface distortions in the form of feet at the base, xe2x80x9ctxe2x80x9d-topping at the top of the resist, and standing waves along the sidewalls of the developed photoresist. This improved developed photoresist can be used to transfer the pattern for the feature to an underlying photomask (reticle).
The present invention pertains to a method of optically fabricating a photomask using a direct write continuous wave laser, which includes the steps of applying an organic antireflection coating over a metal-containing layer; applying a chemically-amplified DUV photoresist, either positive tone or negative tone, over the organic antireflection coating; baking the DUV photoresist at a temperature within a specifically designed range under ambient conditions, with volatile removal assisted by an exhaust hood fan or by similar method (PAB); exposing a surface of the DUV photoresist to radiation from the direct write continuous wave laser; baking the developed photoresist at a temperature within a specifically designed range, again under ambient conditions using an exhausted hot plate (PEB); and, developing the image within the DUV photoresist. Preferably the laser used to image the DUV photoresist is operated at 244 or 257 nm, although other wavelengths may be used. Subsequently, the developed, patterned photoresist is used as a mask for transferring the pattern through a metal-containing layer of the photomask substrate. Typically the pattern transfer is by dry etch. The metal-containing layer of the photomask substrate may include combinations of layers of materials selected from chromium, chromium oxide, chromium oxynitride, molybdenum, molybdenum silicide, and molybdenum tungsten silicide, for example and not by way of limitation.
A preferred antireflection coating (ARC) is organic and may be selected from a negative photoresist containing a DUV dye; a polymeric material prepared from acrylic polymers or copolymers; a binder resin combined with an acid or thermal acid generator and a photoacid generator compound; a binder resin having pendent phenyl groups; and combinations thereof. The organic anti-reflective coating composition preferably comprises acrylic polymers and/or copolymers.
In an alternative embodiment of the method of fabricating a photomask, the ARC may be an inorganic ARC selected to include a material such as chrome oxynitride, titanium nitride, silicon nitride or molybdenum silicide. Further, the ARC used may be a combination of organic and inorganic ARCs. For example, an organic ARC may be applied over the surface of an inorganic ARC.
The post apply bake (PAB) of the photoresist is coordinated with the post exposure bake (PEB) of the photoresist, to obtain both a more uniform critical dimension (CD) of the patterned photoresist across the photomask substrate and a long stability period for a photoresist-coated photomask substrate. The long stability period permits application of the photoresist to the substrate considerably in advance of imaging (exposure) of the photoresist. This permits storage and shipping of the photoresist-coated substrate at the convenience of the manufacturer, prior to actual patterning of the resist.